MANILA, PHILIPPINES — Armed men tried to intimidate ''known opposition voters'' registering for February's presidential election and some ''flying voters'' registered in more than one precinct, a watchdog group charged Saturday.

The National Citizens Movement for Free Elections, or NAMFREL, reported ''confusion and irregularities'' in several regions in the first of two days of voter registration for the Feb. 7 election.

President Ferdinand Marcos, trying to extend his 20-year reign by six years, faces the toughest electoral challenge of his career from opposition candidate Corazon Aquino.

In a report on Saturday's registration, NAMFREL, which received government permission to monitor the polling, said among the complaints it received were ''coercion of known opposition voters not to register by armed men.''

The group also reported ''flying voters'' -- people registering in more than one precinct -- in parts of the capital.

But the government's Commission on Elections, which fielded teams of observers at the country's 86,000 polling stations, reported ''no untoward incidents.''

NAMFREL officials had warned the two days of registration might be the first ''weak link'' in ensuring a clean contest between Marcos and Aquino, whose husband, opposition leader Benigno Aquino, was shot to death at Manila airport in 1983 as he returned from three years of self-imposed exile in the United States.

In the Makati financial district of Manila, Marcos observers were reported posing as inspectors for Aquino, and NAMFREL said its own volunteers were barred from entering several polling stations.

But most of the confusion appeared to stem from identification requirements, which NAMFREL noted also provided an opportunity for fraud.

The election code requires each registrant to supply four photos to register, but many people found it too expensive and in some remote areas photography facilities were unavailable.

In the Manila suburb of San Juan, firefighters were said to offer free photos to those who would vote for Marcos.

NAMFREL said men carrying weapons allowed Marcos backers to register without IDs but charged other registrants $2.60.

Opposition attorneys had appealed to the Supreme Court to waive the photograph requirement, but the panel was in recess and failed to act on the petition.

Opposition assemblyman Jaime Ferrer said the reports of irregularities were apparently ''isolated cases,'' but he warned, ''Tomorrow might be worse. So we're going to watch this thing more carefully.''